1. Technical Field
The present disclosure relates generally to wireless communication systems, and more particularly, to antenna tuner processes in wireless devices.
2. Description of Related Art
In a wireless communication device, a Power Amplifier (PA) is used to drive the transmit signal onto an antenna. Power Amplifiers are designed to operate using a fixed power supply source (e.g., a battery in the wireless communication device) at the maximum efficiency for both the power supply voltage and a known output load, such as 50 ohms. However, in operation, the load impedance experienced by the PA typically varies due to changing environmental conditions. Therefore, a voltage standing wave may be created between the output terminal of the PA and the antenna due to the combination of the forward and reflected powers present in the antenna. As such, the PA may experience a load that is different from the ideal 50 ohm target that the PA was designed for.
A reflection coefficient is commonly used to quantify the severity of the reflections present at the PA output. The reflection coefficient is a complex number including a magnitude component and a phase component, and varies between a magnitude of 0 and 1. A reflection coefficient of 0 indicates that the load is perfectly matched and there is no reflected energy, resulting in maximum power transfer. A reflection coefficient magnitude of 1 indicates that all of the power is reflected. In addition to the mismatch seen by the PA, this mismatch condition creates an insertion loss for the received signal, seen at the input of the receiver Low Noise Amplifier (LNA), and hence adversely affects the noise figure and sensitivity of the receive path.
In advanced transmitter architectures, an Antenna Tuner (AT) circuit is placed between the antenna and the wireless terminal. The AT circuit includes one or more programmable components that can be tuned in order to effect an impedance translation between the antenna and the PA output. This matching of the source impedance to the antenna load impedance enables maximum power transfer from the source to the load. When the source impedance is not matched to the load, some of the transmitted power is reflected away from the antenna. Properly matching the source to the antenna minimizes the power lost due to reflections, and maximizes the power delivered to the antenna (load). In addition, the use of the AT circuit can help minimize the insertion loss in the received signal, which results in an improved noise figure.
One challenge for wireless devices that use AT circuits is to adapt the AT components optimally while minimizing the impact on the transmit channel and/or receive channel. For example, the receive channel seen at the receiver can be characterized by the composite of various convolutions imparted on the received signal, starting with the transmission by the base station. The receiver estimates this composite channel and mitigates its effects. Since the AT circuit is part of this receive channel, if changes are made to the antenna tuner circuit without a corresponding re-estimation of the channel and timing, the receiver performance may degrade.